GENUS RHYNCHONELLA. 
333 
we should hold such views with reservation. In the present instance, I 
must be allowed to doubt whether R. loxia will be found to possess the 
characters of Lower Silurian and of existing Rhynchonella: ; nor do I 
consider the characters of the existing species as congeneric with those 
of the older Silurian or Devonian formations. The necessity felt for some 
other designation to apply to some of these forms has induced the names 
Hypothyris, Hemithyris, Cyclothyris, etc.; but these do not appear to have 
been founded on reliable structural characters. That some subdivision 
will become necessary, and will be adopted, I have no doubt; but-such a 
desirable end can only be consummated after the study of the interiors 
of numerous specimens, with large collections for comparison.* 
Although having adopted the name Rhynchonella for our Devonian species, I 
have lately observed characters which appear to me to separate them so widely 
from the recent species of the genus, that I am compelled to substitute some other 
designation. 
Among genera of the same family, we must necessarily make distinctions upon 
the modifications of certain parts which they may all possess in common • and 
the importance of these modifications of interior parts or appendages should not 
be overlooked. Not only do the recent and fossil forms referred to Rhynchonella 
possess great similarity in the ventral valve, teeth and dental plates, but they are 
not very dissimilar from the same parts in genera of the Spiriferida. It is in the 
dorsal valve and its appendages that we find characters the most important and relia¬ 
ble for generic distinction ; and it is only necessary to follow these in the genera 
of the Terebratulida and Sfiriferida, to recall the most curious and interesting 
modification of the parts which this valve supports. I am therefore inclined to 
regard these modifications of generic importance. 
Ip many of the fossil species referred to Rhynchonella, one of the most con¬ 
spicuous features in the dorsal valve is the strong septum, which becomes broader 
and often shows indications of division at the apex, or at least evidence of a 
small and shallow V-shaped pit. 
* The reliance upon external forms for the determination of generic affinities or differences, although 
a compulsory condition in many instances, is far from satisfactory. As an example of this kind, I may 
remark that having recently occasion to compare Terebratula sappho, Barrande, with Rhynchonella 
sappho, Hall, the former presented so much of a rliynchonelloid aspect as to suggest the propriety of 
regarding the latter as a synonym. A careful examination of the Bohemian species, from cutting down 
the solid fossil, has revealed the fact that it is furnished with calcareous spires arranged as in Atrypa > 
the spires directed into the cavity of the dorsal valve, and the crura connected by a loop in a similar 
manner. 
